The current trend in mobile phone industrial design favors internal antennas, where the antenna is not visible to the customer. The phones include more radio transceivers, for example tri-band UMTS, Quad-band GSM, BT, WLAN, GPS, FM radio, DVB-H, all requiring their own antenna. At the same time there should be room for all the chips on the PCB together with larger display, camera, memory cards, etc., without making the phone appear large and clumsy. Fitting all those antennas into a phone is quite a challenge. The three key parameters when designing mobile phone antennas are bandwidth, size and efficiency. The facts are that a limitation exists with respect to the maximum bandwidth and efficiency obtainable, depending on the realistic size of the antenna. Basically, the minimum bandwidth is determined by the system specification, for example GSM and UMTS, and the efficiency by the total radiated power (TRP) and total isotropic sensitivity (TIS) requirements setup by, for example, CTIA, 3GPP, and mobile operators. The overall size is given by the industrial design. In a standard, non-tunable antenna design it is common to increase the size of the antenna to a level where the requirements for minimum bandwidth and efficiency can be achieved. However, this puts limits on the industrial design and alternatives are desirable.
One approach is to use tunable antennas where the frequency band can be tuned within a system or between bands of different communication systems. In this conventional approach, the antenna only covers a narrow band instantaneously, and the total antenna volume or the number of antennas can be reduced and the selectivity is increased. This conventional approach is well known, but has some limitations in practice.
In a standard antenna design, it is common to increase the size of the antenna to a level where the requirements for minimum bandwidth and efficiency can be achieved and accept the limitations it puts on the industrial design. It is also common to implement a series of decoupling techniques. However, a disadvantage is that these techniques are limited by the physical dimensions of the ground plane.
It is well known that at lower frequencies the mobile phone chassis acts as the main radiator. In fact, the length and the width of the chassis determine univocally the dipole mode of the chassis. The radiating mechanism can be seen as a combination of the antenna and the resonator chassis equivalent resonator forming a system of coupled resonators (as described in Vainikainen, P.; Ollikainen, J.; Kivekas, O.; Kelander, K.; “Resonator-based analysis of the combination of mobile handset antenna and chassis,” Antennas and Propagation, IEEE Transactions on, vol. 50, no. 10, pp. 1433-1444, October 2002). The optimum coupling between the antenna and the chassis happens when the antenna and the chassis resonate at the same resonance frequency. This has the effect of maximizing the impedance bandwidth and increasing the mutual coupling to additional radiators. When the chassis mode is away from the intended resonance frequency of the antenna, the impedance bandwidth will be narrower and the mutual coupling to additional radiators will be lower.
Prior art has always focused on tuning the antenna element itself, varying its electrical length in many different ways (as described in Vainikainen, P.; Ollikainen, J.; Kivekas, O.; Kelander, K.; “Resonator-based analysis of the combination of mobile handset antenna and chassis,” Antennas and Propagation, IEEE Transactions on, vol. 50, no. 10, pp. 1433-1444, October 2002 and K. A. Jose, V. K. Varadan, and V. V. Varadan, Experimental investigations on electronically tunable microstrip antennas, Microw. Opt. Technol. Lett., vol. 20, no. 3, pp. 166169, February 1999).